The notion that matter is made up of small, indivisible
particles goes back to the ancient Greeks. In the sixth century BCE, thinkers
began asking questions about what is the basic underlying reality of the
world. In view of the constant change we see in the world around us, is
there some substratum (physis, hence our word physics) that
is constant? If so, is it material or immaterial, accessible through the
senses or only through the mind, is it one or many? Over the next several
centuries, these questions were answered in several different ways. Some
believed that all was change, others that change was illusory. The Pythagoreans
thought that the physis was "number" and pioneered the mathematical
approach to nature. Their idealist approach was in stark contrast to that
of the materialists, among whom the atomists were most prominent. Leucippus
of Miletus (ca. 435 BCE) and Democritus of Abdera (ca. 410 BCE) developed
the atomic hypothesis. According to them matter can be subdivided only
to a certain point, at which only atoms (that which cannot be cut) remain.
The world is made up of atoms moving in the void. Atoms differed from
each other only in size and shape, and different substances with their
distinct qualities were made up of different shapes, arrangements, and
positions of atoms. Atoms were in continuous motion in the infinite void
and constantly collided with each other. During these collisions they
could rebound or stick together because of hooks and barbs on their surfaces.
Thus, underlying the changes in the perceptible world, there was constancy
(atoms were neither created nor destroyed); change was caused by the combinations
and dissociations of the atoms.

Democritus gave some examples of how the atomic hypothesis
could account for qualities such as color and taste (sharp tastes are
caused by sharp atoms), but on the whole atomism, like other contemporary
global theories, remained a general theory. It was criticized by Aristotle
(384-322 BCE) for some of its logical inconsistencies[1]
and for its inability to explain qualities (color, taste, odor, etc.)
that we call (after Galileo) secondary qualities. Aristotle's matter theory
was fundamentally qualitative: qualities were built into the fundamental
building blocks that made up substances. And against the atomists' idea
of a nature without design or purpose, Aristotle constructed a natural
philosophy that made nature a purposeful agent.

In the philosophical system of Epicurus (341-270 BCE),
physics was subordinated to ethics. The aim of his philosophy was to overcome
irrational fears of natural phenomena and to achieve peace of mind. Epicurus
explained natural phenomena by atomism, but he made several modifications
to the doctrine in view of Aristotle's criticisms. He distinguished between
physical and mathematical divisibility and gave atoms weight. In his system
atoms originally fell through the infinite void with equal speeds, until
one swerved by a tiny amount. This was an uncaused event. This swerve
caused collisions and swirls of atoms, and thus worlds were formed. The
Epicurean ethical system was influential over the next several centuries,
and one of its Roman practitioners, Lucretius (first century BCE), wrote
a long poem about it, De Rerum Natura ("On the Nature of Things"),
from which much of our knowledge about atomism derives.

In the Christian world, nature was seen as the product
of a transcendent creator and was therefore fundamentally rational. Aristotelian
notions of purpose and order fit the Christian mindset much better. Moreover,
in atomism there was an unbridgeable gap between the level of the atoms
and the observable phenomena, whereas Aristotelian natural philosophy
addressed observable phenomena directly. Aristotle did, however, postulate
minima, the theoretical limit of divisibility of substances, and
therefore within European Aristotelianism, there was discussion about
the meaning of this limit and in some quarters minima took on a corpuscular
nature. This train of thought merged with a revived atomism, caused by
the recovery of Lucretius's De Rerum Natura ca. 1415 CE, to give
rise to a corpuscular doctrine that provided the material foundation of
the mechanistic philosophy of the seventeenth century. We must be careful,
however, not to think that all those who sought causal explanations in
the minute building blocks of matter were atomists. Thus, Descartes (1596-1650)
believed that matter was infinitely divisible and had no weight (or mass).

In his Assayer of 1623, Galileo explained his
notion of the difference between those qualities, mostly found by touch,
that are inherent in bodies (weight, roughness, smoothness, etc.) and
those that are in the mind of the observer (taste, color, etc.)--in other
words, the difference between what we call primary and secondary qualities.
In this discussion he referred to bodies that "continually dissolve into
minute particles"[2] and stated his opinion that "for
exciting in us tastes, odors, and sounds there are required in external
bodies anything but sizes, shapes, numbers, and slow or fast movements."[3]
An anonymous cleric filed a report with the Inquisition
in which he claimed the first citation to show that Galileo was an atomist
and the second to be in conflict with the Council of Trent's pronunciations
on the Eucharist.[4] The report did not lead to any action
against Galileo.

Galileo's notions about the constitution of matter
emerge in his Discourses on Two New Sciences of 1638. In his discussion
of cohesion--what holds matter together--he puts forward the notion that
objects are made up of an infinite number of infinitely small particles
held together by an infinite number of small vacua. He did not go beyond
this point, but it is clear that this "atomism" is almost exclusively
mathematical.

Notes:
[1] If atoms have different shapes, then they
have parts, and this means that they are mathematically divisible; if
they have different sizes, then among the infinity of their number there must
be atoms as big as the world.[2] Dtillman Drake and C. D.
O'Malley, The Controversy over the Comets of 1618, (Philadelphia:
University of Pennsylvania Press, 1960), p. 310.[3]Ibid., p. 311.[4] Maurice A.
Finocchiaro,The Galileo Affair: a Documentuary History, (Berkley and
Los Angeles: University of California Press, 1989) pp. 202-204. Pietro
Redondi, Galileo Heretic, (Princeton: Princeton University Press,
1987), pp. 333-35.

Sources: For a synoptic discussion of
the doctrines of Leucippus and Democritus, as well as for Aristotle's criticism
of them, see G. E. R. Lloyd, Early Greek science: Thales to Aristotle
(London: Chatto & Windus; New York, W. W. Norton, 1970). For the
surviving writings of Epicurus, see Epicurus, the Extant Remains, tr.
Cyril Bailey (Hildesheim, New York: G. Olms, 1970). Lucretius, De Rerum
Natura is available in many good translations; Lucretius on the nature
of the universe, tr. R. E. Latham (Harmondsworth: Penguin Books, 1951) is
the most popular version. For an overview of the development of atomism in
early modern Europe, see Robert H. Kargon, Atomism in England from Harriot
to Newton (Oxford: Clarendon Press, 1966). For Galileo and atomism, see
Pietro Redondi, Galileo Heretic, tr. Raymond Rosenthal (Princeton:
Princeton University Press, 1987). For Galileo's discussion of vacua and
particles, see Two New Sciences, tr. Stillman Drake (Madison: University
of Wisconsin Press, 1974), pp. 19-34. For the passage on atoms in Galileo's
Assayer, see Stillman Drake and C.D. O'Malley, The Controversey over
the Comets of 1618 (Philadelphia: University of Pennsylvania Press,1960),
pp. 310-311.